1. Field of the Invention
The present invention relates to a magnetic disc, and more particularly, it relates to a magnetic disc with improved lubricity comprising a non-magnetic support substrate and a magnetic layer consisting of a magnetic coating which is formed on at least one surface, preferably on both surfaces of the substrate and comprises ferromagnetic powder and a binder.
2. Discussion of Related Art
It has been proposed and practically employed to add a liquid lubricant to a magnetic coating of an audio tape or a video tape so as to decrease sliding friction between a magnetic coating surface and a magnetic head. For the video tape and the like, since it has better lubricating properties than other liquid lubricants, an aliphatic acid ester is used in order to satisfy both lubricity and strength of the magnetic layer in relation with the so-called steel characteristic which relates to repeated sliding contact of the magnetic coating with the magnetic head.
In this connection the following Patent Publications can be referred to:
Japanese Patent Publication Nos. 23899/1968, 12950/1972, 14648/1972, 26882/1972, 13042/1973, 15002/1973, 2414/1983 and 23008/1984, Japanese Patent Kokai Publication 117735/1980, 117739/1980, 130435/1980 and 173835/1980 and U.S. Pat. No. 4,465,737.
The addition of the aliphatic acid ester to the magnetic coating satisfactorily increases lubricity, strength and durability of the magnetic coating of the video tape and the like.
Recently, a floppy disc is increasingly required to have better lubricity of the magnetic coating and durability and strength of the magnetic layer.
The reasons for such requirements are as follows:
1. In case of the floppy disc, since a search and transmission time of data is shortened with the development of peripheral equipment for recording and reproducing, it is necessary to rotate the floppy disc at a high rate; PA0 2. The floppy disc is slidingly forced against both its surfaces in such a state that it is held between a pair of the magnetic heads or between a magnetic head and a support member; PA0 3. To increase a linear recording density, the thickness of the ferromagnetic coating layer should be decreased; PA0 4. A so-called metal floppy disc which contains, for example, metal iron powder as the ferromagnetic powder has a softer coating than a floppy disc which contains a metal oxide as the ferromagnetic powder so that it has a larger coefficient of friction when slidingly contacted with the magnetic head.
For the above reasons, more improved and different lubricating properties are required for the magnetic coating of the video tape or the floppy disc.
Since a relative sliding speed between the magnetic head and the surface of the magnetic coating is increased, heat is generated at the contact part between the two due to sliding so that the binder resin contained in the magnetic coating thermally deteriorates. Therefore, the magnetic coating is severely damaged by the deterioration of the binder and sliding force.
In view of the above, as a result of extensive study by the present inventors to develop a magnetic disc satisfying both good lubricity and strength, namely durability of the magnetic coating, the following facts were found:
In order to improve the lubricity of the magnetic coating, use of a large amount of a liquid lubricant is particularly effective, since the liquid lubricant has good compatibility with the binder resin contained in the magnetic coating so that it can be contained in the magnetic layer in a larger amount. However, since the binder resin attracts a large number of the lubricant molecules, it swells so that its inherent binding force for the magnetic powder is decreased, which results in deterioration of the magnetic layer.
Since the durability of the magnetic layer is deteriorated, if the amount of the lubricant is greatly increased it is desirable for the lubricant to be locally contained near the surface layer of the magnetic coating which is in sliding contact with the magnetic head and not homogeneously throughout the magnetic coating.
Further investigation was carried out by the inventors based on the above findings to reveal the following:
When a solid aliphatic acid ester was used as a lubricant, even if it was homogeneously dispersed in the magnetic coating, it migrated toward the surface of the magnetic coating because it has a poor compatibility with the binder resin as compared with the liquid lubricant and a smaller specific gravity than the binder resin and the magnetic powder.
Thus, a magnetic disc utilizing a solid aliphatic acid ester as the lubricant was produced and examined to find that it had good lubricity and durability. However, after such a magnetic disc was stored for a long period of time, its lubricity and durability were again examined to find that the lubricity greatly deteriorated.
To clear up the cause of the deterioration of the lubricity, the surface of the magnetic coating was carefully studied. As a result, white powder was found to be present on the surface of the magnetic coating. The analysis of the white powder revealed that it was separated solid aliphatic acid ester.
As a result of further study to prevent such separation of the aliphatic acid ester, it has been concluded that some stabilizer must be added to the magnetic coating, in addition to the binder resin and the solid aliphatic acid ester, which is locally present near the surface of the coating.